1. Field of the Invention
The present invention relates to the removal of metal ions and/or their complexes from a solution.
2. Description of the Related Art
Metal ions and/or their complexes, hereinafter collectively referred to as "metal ions", can be removed from waste water streams by using precipitating agents which form insoluble compounds. See, for example, published Japanese Patent Application Nos. 07-148492 and 07-248582, European Patent Application Publication No. 0670289 A1, and "Silver Recovery from Photographic Waste Processing Solutions By Using the Trisodium Salt of 2,4,6-Trimercapto-5-Triazine," by Spears et al., presented at the Seventh International Symposium on Photofinishing Technology in San Francisco, Calif., Feb. 3-5, 1992, the disclosures of which are herein incorporated by reference.
Conventional precipitation technology uses sequential processing. Referring to FIGS. 1a and 1b, for example, it is shown in block diagram and schematic format, respectively, a conventional processing system 10 for removing metal ions from waste water. System 10 includes an influent line 12 for providing waste water to a mixing unit 14 for mixing and reacting the precipitating agent with the metal ions contained in the waste water and also for conducting sedimentation of the precipitate.
Treated solution flows through line 16 by the pumping action of pump 32 to a sludge collection unit 18, which may include a coarse filter for removing relatively large particles and a fine filter for removing relatively small particles from the solution; the coarse and fine filters are sequentially arranged. Filtrate from sludge collection unit 18 flows through line 20 to discharge unit 22, and a portion of the filtrate may optionally be returned through recycle line 24 to the mixing unit 14 to further react or coat dead-end type filters to increase system efficiency.
This system is shown in more detail in FIG. 1b. In particular, mixing unit 14 typically comprises a tank 26 including a stirrer 28. Treated solution flows out from the system through discharge line 40. Optional recycle lines 42 and 35 may be used to coat the filtration media 34 and 38 respectively in the initial stages.
The above-described sequential system, however, is disadvantageous. The most significant disadvantage is the sludge run-through problem. In order to achieve reasonable processing capacity, the system throughput must be maintained at a certain level. However, at this level, the flux of the solution passing through the filter tends to be so large that it may breakdown the precipitate, causing sludge to run through the filters to the discharge. This is especially true for soft polymeric precipitates.
While it may be possible to solve this problem by increasing the effective area of the filtration medium to reduce the actual flux while keeping the same level of total throughput, it will be appreciated that this solution is neither desired from an economic standpoint, nor from the standpoint of maintaining a consistently reliable and very low silver discharge level.
Additionally, the above-described sequential technique for removing metal ions from a waste water stream is typically conducted as a batch process which involves both time consuming charging and discharging steps plus a sedimentation step. Still further, this system is disadvantageous economically when scaling up, since the capacity and size of the equipment will be proportional to the treatment capacity desired.